Circlips (also known as retaining rings) are fasteners that resemble flexible, open-ended rings. Circlips are usually made of stainless steel or similar metals. 

In an assembly, Circlips or Retaining Rings serve as a removable shoulder for retaining components in a bore or on a shaft. The external flat spring ring splits at a particular point, allowing it to be sprung open and fitted around a bore or housing before closing tightly into a recess around the shaft. Internal circlips, on the other hand, can be closed to fit inside a shaft before springing back into shape and forming a shoulder in the housing.